1. Field of the Invention
This invention generally relates to a thin film depositing technology and more particularly relates to a method for depositing microcrystalline silicon.
2. Description of the Related Art
A tandem silicon thin film solar cell generally uses microcrystalline silicon which has an advantage to increase photoelectric conversion efficiency of solar cells. The microcrystalline silicon is generally fabricated by plasma enhanced chemical vapor deposition (PECVD). However, the deposition rate of the microcrystalline silicon is low (about 2 Å/sec), thus becoming a bottleneck during application. Specifically, a thin film solar cell generally requires a microcrystalline silicon having thickness of about 1-2 μm, such that the depositing time often exceeds one hour. Such low deposition rate is more problematic for large area depositing processes because crystallinity uniformity is concerned. Therefore, the low throughput of depositing microcrystalline silicon resulting in the cost of generating electric power per watt is high. Accordingly, it is important to increase depositing rate under good crystallinity of microcrystalline silicon.
A conventional art, Japanese patent JP 20030421313, uses different duty time of pulse plasma in deposition time to increase depositing rate and crystallinity ratio of microcrystalline silicon. However, the multi-step process setting have made the process operation more complicated and difficult. Further, Japanese patent JP 20030421313 discloses an open loop process which cannot in-time modulate process condition according to monitored variation of species (e.g. SiHx and H) in a plasma. Therefore, the depositing rate of this art might be limited due to the excessive hydrogen etching effect while the hydrogen species continually increases during deposition process.